1. Field of the Invention
The present invention relates to a distance measuring device which measures a distance from a camera to an object to be photographed by use of light-passive method.
2. Description of the Related Art
In distance measuring devices which detect a distance to an object to be photographed, it is conventionally known that a focus detecting device which detects a focusing condition of an objective lens of a camera can be used. Among such focus detecting devices there is a type which analyzes an image signal obtained by an image sensor, and it is difficult to accomplish a reliable focus detection when a contrast of an image on the image sensor is lower than the predetermined level.
When a line image sensor is used as an image sensor, even though focus detection can not be executed if a image on the line image sensor has an low contrast, it often occurs that the contrast of the image in the direction perpendicular to the line-sensor is high sufficient. When portraits or scenes are photographed, the contrast in the horizontal direction is often higher than that in the perpendicular direction. Then it is reasonable to arrange an image sensor in the horizontal direction if a line image sensor is used for focus detection.
In such arrangement, however, focus detection can not be executed when an object to be photographed has a low contrast in the horizontal direction and high-contrast in the perpendicular direction. Also, the same problem occurs when a camera is used in a vertical orientation. This problem can be solved by using a two-dimensional image sensor.
Japanese Laid-Open Patent Publication No. 59-174807 discloses this kind of focus detecting device with a two-dimensional image sensor. Mainly it discloses that an object image is to be formed on photo sensors arranged in two-dimensions, and when the contrast is not sufficient as a result of reading out the output of photo sensors in one direction at the arrangement of the photo sensors, the output of photo sensors in the other direction is read out. In this way, calculation for focus detection can be achieved by searching for a direction with a sufficient contrast. However, the focusing position or distance can not be previously obtained before photographing because it is a contrast method. Besides, the device is expensive.
Further, especially when an object image is formed separately on an expected focal plane by an object luminance which passes through two zones of a lens divided by an optical axis, each object image is deviated because of a parallax to an object of said two zones on the lens. In a focus detecting device that detects and calculates the amount of deviation, it is difficult to obtain a reliable amount of the deviation as to an object with poor contrast in a row direction of said two zones. Even if an image sensor with two-dimensional resolution may be used, this problem can not be solved.
Further, in Japanese Patent Application No. 61-57855, a focus detecting device has been proposed having a purpose of obtaining a reliable focus detecting for any objects. This is done by using a line image sensor and solving the problems associated with a distribution of an object's contrast becomes having a directional characteristic, especially in a system which carries out a focus detection by use of an object light which passes through two different zones of a lens.
Following is an explanation of the construction of the focus detecting device and the inconveniences of the same. FIG. 1 shows a single-lens reflex camera with a focus detecting device. A luminous flux, which passes through an exit pupil of an interchangeable lens (a photographing lens) 1, reflects at a sub-mirror 4 which is fixed to a backside of a main mirror 3 in a mirror box 2, and the luminous flux is guided to an AF (Auto, Focus) module 5.
There are two restrictions for distance measuring on this construction. One is a limitation for having an AF luminous flux vary widely because of an existence of a group of the interchangeable lenses and the other is that distance measuring range is restricted because of a limitation for enlarging the size of the sub-mirror.
FIG. 2 is a view explaining the limitation of an AF detecting zone by an exit pupil of a group of interchangeable lenses. In the figure, a vertical axis shows a length of light-receiving unit of an AF sensor, and a lateral axis shows an optical axis of a photographing lens, and hatched zone shows an envelope curve (envelope cone) in arranging the exit pupil of the interchangeable lens. In a phase-difference detecting method, if luminous flux for auto-focusing is eclipsed only in its part, focus detection can not be executed because a symmetry of two images does not exist and can not be compared. In this way an accurate focus detection can not be executed unless the luminous flux for auto-focusing lies inside line "a" of FIG. 2. That is, the size of focusing area must be from 2 to 3 millimeter (mm) from an optical axis (length of area must be from 4 to 6 mm).
The sub-mirror 4 is located at the main mirror 3 within the mirror box 2. A light nearby an optical axis of the photographing lens 1 is reflected for luminous flux for auto-focusing, but a light around the optical axis is not taken into account. It is necessary to arrange the sub-mirror 4 whose size is the same as the main mirror 3 in the perpendicular angle to the upper edge of the main mirror 3, but then the sub-mirror 4 can not be contained in the mirror box 2 unless the mirror box 2 is enlarged to twice its present size. Also, in this case, although a backside (lens back) of a photographing lens 1 has a predetermined value, it is not possible to maintain the value. That is, a single-lens reflex camera can not be constructed in this condition. According to the limitation of the size of the sub-mirror 4, the length of focusing area in vertical direction is restricted within a few mm.
Further, in conventionally known TTL type such as single-lens reflex camera which executes focus detection by luminous flux that passes through a photographing lens, there is a device in which sensors consisting an AF module is provided not only in horizontal direction but also in vertical direction in cross-pattern, so that focus detection for an object with no contrast in horizontal direction can be also achieved by use of a vertical image sensor.
Now, a line-sensor should be arranged in both horizontal and vertical directions all over a finder screen in order to raise the probability of an AF detecting. In the TTL type, however, the line-sensor can not be extended in the vertical direction because of the limitation by an interchangeable lens (f-value of an exit pupil) and the limitation of the sub-mirror's size (conventionally, the length of the sensor in vertical direction is restricted within 2 to 3 mm for 24 mm of a finder in a longitudinal direction).
FIG. 3 shows a principle arrangement view of an optical system and an AF module in cross-pattern (an image sensor is used) in a TTL phase-difference detecting method of a conventional single-lens reflex camera. FIG. 4 shows an AF module of a TTL phase-difference detecting method. In FIG. 3, an optical system includes a photographing lens 1 of a camera, a visual field mask 7 with a cross-pattern aperture, a condenser lens 8, a group of four imaging lenses 9, and a image sensor 10. Dotted circles (a, b) on the photographing lens 1 are projected images through the a pupil mask, which is located in front of the imaging lens 9, through the condenser lens 8, of the imaging lens 9. The surface of the visual field mask 7 is the expected focal plane of the photographing lens 1, and located on equivalent to a film plane of the camera. The imaging lens 9 forms an image of the visual field mask 7 on the image sensor 10.
In the above-mentioned structure, an image formed on the visual field mask 7 by an object luminance which passes through an area (a) on the photographing lens 1 is formed on the image sensor 10 by the imaging lens 9(a'). In the same manner, an image formed on the visual field mask 7 by an object light which passes through an area (b) on the photographing lens 1 is formed on the image sensor 10 by the imaging lens 9(b'). Rectangles X1 and X2 shown in cross-pattern on the image sensor 10 are images of horizontal part of a cross-pattern aperture on the visual field mask 7 by a pair of imaging lens (a') and (b') in horizontal direction. In the same manner, Rectangles Y1 and Y2 is an image of vertical part of cross-pattern aperture on the visual field mask 7. On rectangles X1 and X2, the same portion of an object image is formed on X1 and X2 respectively, provided that an image which passes through the photographing lens 1 is formed on the visual field mask 7, that is in-focus situation, and making the re-imaged image's position on X1 and X2 of the same portion of the object as a reference point. The re-imaged images on X1 and X2 approach each other when the object image is formed on a near side to the photographing lens 1 than the visual field mask 7 (front-focus). The re-imaged images on X1 and X2 move apart from each other when the object image is formed on rear side of the field mask 7 (rear-focus). Therefore, the calculation for which side and how much an object image to be photographed deviates from the correct focus position can be done by the following steps; arranging line image sensors in the direction of series of X1 and X2, gradually shifting and folding image signals of the object image on X2 for an image signal of the object signal on X1 in processing operation of image signals, and detecting the amount of the shift that shows the maximum in correlation of both image signals. The above-mentioned explanation is a principle of focus detection. The processing operation of image signals is disclosed in Japanese Laid-Open Patent Publication No. 60-247210.
According to the above-mentioned principles, line-sensors are arranged on the image sensor 10 along the lines of X1, X2, and the lines of Y1, Y2 at a perpendicular angle to the lines X1 and X2. In FIG. 4, a photographing lens is out of the figure. A transmitting light of the photographing lens is directed to an infrared ray-cut filter 11, a visual field mask 7, and a condenser lenses 8 by the sub-mirror 4 in FIG. 1, and directed to a horizontal direction by the mirror 12 in 45.degree. angle, further, projected on the image sensor 10 through the pupil mask 13, and the imaging lens 9 (4 mirrors in two pairs). All of the above-mentioned elements are unified in a unit as an AF module by a frame 14. As above-mentioned, the image sensor 10 is composed of line image sensors arranged in horizontal and vertical direction, and CCD image sensor is used as a line image sensor.
FIG. 5 is a principle view of one line of an AF module for lens shutter (LS) camera in a conventional light-passive phase-difference detecting method. As for a principle of detecting an object distance L, a variation of intervals between object images projected on photosensor arrays 21, and 22 are obtained. The distance L is obtained by use of a formula, L=Bf/(X1+X2), defining a focal length as "f", a base length of photosensor arrays 21 and 22 as "B", and positions of an object image in distance L projected on the photosensor arrays 21 and 22 as "X1" and "X2".
FIG. 6 (a) is a front view showing a detail structure of the above-mentioned AF module, and (b) is a sectional side view showing the same. The AF module consists of a separator lens 23 for obtaining two object images, a mask 24 for cutting down stray light, an AF sensor 25 (sensor 22 in FIG. 5) with two line-sensors for obtaining two images, and a holder 26 for holding these members. In the above-mentioned AF module for lens shutter camera, since the photosensor arrays 21 and 22 are provided only in a horizontal direction, high reliability of focusing can not be achieved.